273 to Slant Six Conversion?

Slickchick is correct. K-frame's the same through '66, slant-6 or V8, so all you need is the '62-'66 slant-6 engine mount brackets, and a pair of these. The radiator needs the bottom hose connection on the passenger side. You'll have some adaptation to do on the engine wiring harness (extend a few wires). Can't think of anything major other than that, it's an easy swap.